193nm laser and inspection system

ABSTRACT

A laser for generating an output wavelength of approximately 193.4 nm includes a fundamental laser, an optical parametric generator, a fourth harmonic generator, and a frequency mixing module. The optical parametric generator, which is coupled to the fundamental laser, can generate a down-converted signal. The fourth harmonic generator, which may be coupled to the optical parametric generator or the fundamental laser, can generate a fourth harmonic. The frequency mixing module, which is coupled to the optical parametric generator and the fourth harmonic generator, can generate a laser output at a frequency equal to a sum of the fourth harmonic and twice a frequency of the down-converted signal.

PRIORITY APPLICATION

This application is a continuation of U.S. patent application Ser. No.14/158,615 entitled “193 NM LASER AND INSPECTION SYSTEM” filed Jan. 17,2014 which claims priority of U.S. Provisional Patent Application61/756,209, filed on Jan. 24, 2013 and incorporated by reference herein.

RELATED APPLICATION

The present application is related to U.S. patent application Ser. No.13/797,939, entitled “Solid-State Laser and Inspection System Using 193nm Laser”, filed on Mar. 12, 2013 by Chuang et al. and incorporated byreference herein.

BACKGROUND OF THE DISCLOSURE

1. Field of the Disclosure

This disclosure relates to a laser and specifically to a solid state orfiber laser that generates radiation near 193 nm and is suitable for usein inspection of photomasks, reticles, and/or wafers. The laser may bepulsed (Q-switched or mode-locked) or CW (continuous wave).

2. Related Art

Excimer lasers for generating light at 193 nm are well known in the art.Unfortunately, such lasers are not well suited to inspectionapplications because of their low laser pulse repetition rates and theiruse of toxic and corrosive gases in their lasing medium, which leads tohigh cost of ownership.

Solid state and fiber lasers for generating light near 193 nm are alsoknown. Exemplary lasers use two different fundamental wavelengths or theeighth harmonic of the fundamental, either of which requires lasers ormaterials that are expensive or are not in high volume production.Moreover, most of these lasers have very low power output and arelimited to laser pulse repetition rates of a few MHz or less.

Therefore, a need arises for a laser and preferably to a solid state orfiber laser that generates radiation near 193 nm and is suitable for usein inspection of photomasks, reticles, and/or wafers. Notably, suchinspections at high speeds often require minimum laser pulse repetitionrates of multiple MHz (e.g. greater than 50 MHz in some cases).

SUMMARY OF THE DISCLOSURE

A laser for generating an output wavelength of approximately 193.4 nm isdescribed. This laser includes a fundamental laser, an opticalparametric generator, a fourth harmonic generator, and a frequencymixing module. The optical parametric generator, which is coupled to thefundamental laser, can generate a down-converted signal. The fourthharmonic generator, which is coupled to the optical parametricgenerator, can generate a fourth harmonic. The frequency mixing module,which is coupled to the optical parametric generator and the fourthharmonic generator, can generate a laser output of a frequency equal toa sum of the fourth harmonic and twice a frequency of the down-convertedsignal. Notably, the frequency mixing module comprises two non-linearcrystals. In one embodiment, a first non-linear crystal is configured togenerate a frequency equal to a sum of the fourth harmonic and thefrequency of down-converted signal by type-II conversion, and a secondnon-linear crystal is configured to generate the frequency equal to thesum of the fourth harmonic and the twice the frequency of thedown-converted signal by type-I conversion.

Another laser for generating an output wavelength of approximately 193.4nm is described. This laser includes a fundamental laser, first andsecond frequency doubling modules, an optical parametric generator, anda frequency mixing module. The first frequency doubling module, which iscoupled to the fundamental laser, can generate a second harmonic. Thesecond frequency doubling module, which is coupled to the firstfrequency doubling module, can generate a fourth harmonic. The opticalparametric generator, which is coupled to the first frequency doublingmodule or the fundamental laser, can generate a down-converted signal.The frequency mixing module, which is coupled to the optical parametricgenerator and the second frequency doubling module, can generate a laseroutput of a frequency equal to a sum of the fourth harmonic and twice afrequency of the down-converted signal. Notably, the frequency mixingmodule comprises two non-linear crystals. In one embodiment, a firstnon-linear crystal is configured to generate a frequency equal to a sumof the fourth harmonic and a frequency of the down-converted signal bytype-I conversion, and a second non-linear crystal is configured togenerate the frequency equal to the frequency of the sum of the fourthharmonic and the twice the down-converted signal by type-II conversion.

Yet another laser for generating an output wavelength of approximately193.4 nm is described. This laser includes a fundamental laser, afrequency doubling module, a frequency combiner, an optical parametricgenerator, and a frequency mixing module. The frequency doubling module,which is coupled to the fundamental laser, can generate a secondharmonic. The frequency combiner, which is coupled to the frequencydoubling module, can generate a third harmonic. The optical parametricgenerator, which is coupled to the frequency doubling module or thefrequency combiner, can generate a down-converted signal. The frequencymixing module, which is coupled to the optical parametric generator andthe frequency combiner, can generate a laser output of a frequency equalto a sum of the third harmonic and twice a frequency of thedown-converted signal.

These 193.4 nm lasers can be constructed using components that arereadily available and are relatively inexpensive. For example, thefundamental laser used in the various described embodiments can generatea fundamental frequency of approximately 1064.3 nm, approximately 1053nm, approximately 1047 nm, or approximately 1030 nm. These fundamentallasers are readily available at reasonable prices in variouscombinations of power and repetition rate. The fundamental laser caninclude a laser diode or a fiber laser.

The optical parametric generator, implemented as an optical parametricamplifier (OPA) or as an optical parametric oscillator (OPO), caninclude a periodically poled non-linear optical crystal. Exemplaryperiodically poled non-linear crystals can be formed from lithiumniobate (LN), magnesium-oxide doped lithium niobate (Mg:LN),stoichiometric lithium tantalate (SLT), magnesium-oxide dopedstoichiometric lithium tantalate (Mg:SLT), or potassium titanylphosphate (KTP).

The down-converted signal generated by the optical parametric generatorhas a signal wavelength of one of approximately 1380 nm to 1612 nm, 1416nm, 818 nm to 918 nm, and 846 nm to 856 nm.

The frequency mixing module can include a cesium lithium borate (CLBO)crystal, a beta barium borate (BBO) crystal, or a lithium triborate(LBO) crystal. In one exemplary mixing technique, the fourth harmonic ata wavelength of approximately 266 nm is mixed with the down-convertedsignal at approximately 1416 nm (infra-red light) to generate awavelength of approximately 224 nm. The approximately 224 nm light isthen recombined with the down-converted signal to generate a wavelengthof approximately 193 nm. These two frequency mixing stages contribute tothe overall high efficiency and stability of the 193 nm laser. In somepreferred embodiments, these two frequency mixing stages can includeCLBO crystals, which at a temperature near 100° C., can perform thesetwo conversions with high efficiency (e.g. non-linear coefficients canbe approximately 0.5 to 1 pm V⁻¹) and small walk-off angles. In oneembodiment, type II mixing in CLBO can be used for the conversion stagethat generates the approximately 224 nm wavelength followed by type-Imixing in CLBO to generate the approximately 193 nm wavelength. Inanother embodiment, type-I mixing in CLBO can be used for the conversionstage that generates the approximately 224 nm wavelength followed bytype-II mixing in CLBO to generate the approximately 193 nm wavelength.In some embodiments, one or both of these two frequency mixing stagesmay be performed using a non-linear optical crystal other than CLBO,such as BBO (beta barium borate) or LBO (lithium triborate).

The improved lasers for generating an output wavelength of approximately193.4 nm described herein can be continuous-wave lasers, Q-switchedlasers, mode-locked lasers, or quasi-continuous-wave lasers. Compared toeighth harmonic lasers, these improved lasers are significantly lessexpensive, and have longer life and better cost of ownership. Moreover,compared with low repetition rate lasers, these improved lasers cansignificantly simplify the illumination optics of the associatedinspection system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, and 1C show block diagrams of exemplary improved 193 nmlasers.

FIG. 1D shows a table of exemplary wavelength ranges for the improved193 nm lasers shown in FIGS. 1A and 1B.

FIG. 1E shows a table of exemplary wavelength ranges for the improved193 nm laser shown in FIG. 1C.

FIG. 2 shows one embodiment of a fourth harmonic generator.

FIG. 3 shows one embodiment of a frequency mixer module.

FIGS. 4A and 4B show some embodiments of frequency mixers that can beused in the improved 193 nm lasers.

FIG. 5 illustrates an exemplary amplifier module in which a seed lasercan generate stabilized, narrow-band seed laser light.

FIG. 6 illustrates an exemplary OPO/OPA configured to generate thedown-converted signal at frequency ω_(s).

FIG. 7 shows a reticle, photomask, or wafer inspection system thatsimultaneously detects two channels of image or signal on one sensor.

FIG. 8 illustrates an exemplary inspection system including multipleobjectives and one of the above-described improved 193 nm lasers.

FIG. 9 illustrates the addition of a normal incidence laser dark-fieldillumination to a catadioptric imaging system.

FIG. 10A illustrates a surface inspection apparatus that includes anillumination system and a collection system for inspecting areas of asurface.

FIG. 10B illustrates an exemplary array of collection systems for asurface inspection apparatus.

FIG. 11 illustrates a surface inspection system that can be used forinspecting anomalies on a surface.

FIG. 12 illustrates an inspection system configured to implement anomalydetection using both normal and oblique illumination beams.

FIG. 13 illustrates an exemplary pulse multiplier for use with theabove-described improved 193 nm laser in an inspection or metrologysystem.

FIG. 14 illustrates a coherence reducing subsystem for use with theabove-described improved 193 nm laser in an inspection or metrologysystem.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a block diagram of an exemplary improved 193 nm laser100A. In laser 100A, a fundamental laser 102 operating at a frequency ωcan generate fundamental light 128 (called the “fundamental” in theindustry). In one embodiment, the frequency ω (the fundamental) maycorrespond to an infra-red wavelength near 1064 nm. Note that when awavelength is used herein without qualification, such wavelength refersto the vacuum wavelength of the light. An exemplary fundamental laser102 can be implemented by a laser using a Nd:YAG (neodymium-dopedyttrium aluminum garnate) lasing medium or a Nd-doped yttriumorthovanadate lasing medium or by an ytterbium-doped fiber laser.Suitable fundamental lasers are commercially available as pulsed(Q-switched or mode-locked) or CW (continuous wave) from Coherent Inc.(including models in the Paladin family with repetition rates of 80 MHzand 120 MHz), Newport Corporation (including models in the Explorerfamily) and other manufacturers. Laser power levels for such fundamentallasers can range from milliWatts to tens of Watts or more.

In laser 100A, the fundamental 128 is directed towards an opticalparametric generator, e.g. an optical parametric oscillator or anoptical parametric amplifier (OPO/OPA) 116. OPO/OPA 116 can down-convertpart of the fundamental light 128 to a down-converted signal 129 offrequency ω_(s). In some preferred embodiments, the wavelengthcorresponding to ω_(s) is approximately 1416 nm (e.g. within a rangefrom about 1330 nm to about 1612 nm, within a range from about 1378 nmto 1461 nm, or within a range from about 1413 nm to about 1417 nm).

In some embodiments, only part of the fundamental 128 is consumed in thedown-conversion process. In such embodiments, the unconsumed part of thefundamental light 128, i.e. the unconsumed fundamental 130, is directedto a fourth harmonic generation module 103. Module 103 (described infurther detail below) typically includes multiple frequency conversionstages to generate the 4^(th) harmonic (4ω) from the unconsumedfundamental (ω). This 4^(th) harmonic (4ω) can be combined with thedown-converted signal 129 in a frequency mixing module 118 to create alaser output 140 having a frequency substantially equal to the sum of 4ωand 2ω_(s). In some embodiments, the output wavelength of the laseroutput 140 is substantially equal to 193.368 nm. In other embodiments,the output wavelength is between approximately 190 nm and 200 nm orbetween approximately 192 nm and 195 nm.

FIG. 1B shows a block diagram of an alternative improved 193 nm laser100B. In this embodiment, the fundamental (ω) (generated by any of theabove-mentioned lasers) is directed to a first frequency doubling module110 to generate a second harmonic (2ω). The unconsumed fundamentaloutput by the first frequency doubling module 110 can be directed to theOPO/OPA 116, which in turn generates the down-converted signal ω_(s). Asnoted above, the wavelength of the down-converted signal ω_(s) isapproximately 1416 nm (e.g. within a range from about 1330 nm to about1612 nm, within a range from about 1380 nm to 1461 nm, or within a rangefrom about 1413 nm to about 1417 nm).

The second harmonic (2ω) generated by the first frequency doublingmodule 110 is directed to a second frequency doubling module 112 togenerate a fourth harmonic (4ω). The frequency mixing module 103 cancombine the fourth harmonic (4ω) and the down-converted signal (ω_(s))to create the laser output 140 having a frequency approximately equal tothe sum of 4ω and 2ω_(s). As noted above, in some embodiments, thisoutput wavelength is approximately equal to 193.368 nm. In otherembodiments, the output wavelength is between approximately 190 nm and200 nm or between approximately 192 nm and 195 nm.

In yet another embodiment, the fundamental (ω) output by the fundamentallaser 102 can be split into two portions. One portion is directed to thefirst frequency doubling module 110, and the other portion is directedto the OPO/OPA 116 (shown by arrow 109). Thus, in this embodiment, theunconsumed fundamental ω output by the first frequency doubling module110 is not directed to OPO/OPA 116. The second frequency doubling module112 and the OPO/OPA 116 are both still coupled to the frequency mixingmodule 103, as shown in FIG. 1B. Substantially similar wavelengths areused and generated in this modified embodiment as in the above-describedimproved 193 nm laser 100B.

FIG. 1D shows a table of exemplary wavelength ranges (in nm) for theimproved 193 nm lasers shown in FIGS. 1A and 1B. For each fundamentallaser type, an exemplary short-wavelength fundamental and an exemplarylong-wavelength fundamental are shown, along with the wavelengthscorresponding to the harmonics and the down-converted signal requiredfor the desired output wavelength (193.4 nm in the example shown in thetable). The exact wavelength of a fundamental laser depends on manyfactors including the exact composition of the lasing medium, theoperating temperature of the lasing medium, and the design of theoptical cavity. Two lasers using the same laser line of a given lasingmedium may operate at wavelengths that differ by a few tenths of 1 nm ora few nm due to the aforementioned and other factors. One skilled in theappropriate arts would understand how to choose the appropriatewavelength for the down-converted signal in order to generate thedesired output wavelength from any fundamental wavelength close to thoselisted in the table. Similarly, if the desired output wavelength differsfrom 193.4 nm by a few nm, the desired output wavelength can also beachieved by an appropriate adjustment of the wavelength for thedown-converted signal.

FIG. 1C shows another alternative embodiment of an improved 193 nm laser100C. In this embodiment, the fundamental ω output by the fundamentallaser 102 is directed to the frequency doubling module 110 to generate asecond harmonic 2ω. The fundamental ω may be generated by any of theabove-mentioned lasers. The portion of the fundamental ω not consumed inthe frequency doubling module 110 and the second harmonic 2ω are thendirected to a frequency combiner 133, which generates a third harmonic3ω.

The second harmonic 2ω not consumed by the frequency combiner 133 isthen directed to an OPO/OPA 136 to generate a down-converted signal 132having a frequency ω_(s). In one modified embodiment (shown by arrow140), a portion of the second harmonic 2ω can be taken directly from theoutput of the frequency doubling module 110 and directed to the OPO/OPA136 (i.e. instead of taking the unconsumed second harmonic from theoutput of the frequency combiner 133). In some embodiments, thewavelength of the down-converted signal 132 is approximately 850 nm(e.g. within a range from about 818 nm to about 918 nm, or within arange from about 836 nm to 867 nm, or within a range from about 846 nmto about 856 nm). Note that the exact wavelength selected for ω_(s)depends on the exact wavelength of the fundamental laser and the desiredoutput wavelength. Different fundamental lasers of the same type maydiffer in wavelength by a few tenths of a nm due to different lasingmedium temperatures, lasing medium composition variations, and othersmall differences in the laser cavity design. In some embodiments, anoutput wavelength within a few nm of 193 nm is possible using theimproved 193 nm laser, such as an output wavelength between 192 nm and195 nm or between 190 nm and 200 nm.

A frequency mixing module 138 can combine the third harmonic 3ω and thedown-converted signal 132 (ω_(s)) to create a laser output 150 at awavelength that corresponds to a frequency substantially equal to thesum of 3ω and 2ω_(s). In some embodiments, this output wavelength issubstantially equal to 193.368 nm. In other embodiments, this outputwavelength is between approximately 190 nm and 200 nm or betweenapproximately 192 nm and 195 nm. In some embodiments, the frequencymixing module 138 may include a CLBO crystal to mix 3ω and ω_(s) tocreate the sum of these two frequencies at a wavelength of approximately250 nm (when ω corresponds to a wavelength of approximately 1064 nm). Insome embodiments, the frequency mixing module 138 may further include aBBO, KBBF (Potassium beryllium fluoroborate) or KBO (potassiumpentaborate tetrahydride) crystal to mix the sum of 3ω+ω_(s) with ω_(s)to create the laser output 150 (corresponding to a wavelength ofapproximately 193.4 nm in one preferred embodiment). FIG. 1E shows atable of exemplary wavelength ranges (in nm) for the improved 193 nmlaser shown in FIG. 1C. FIG. 1E shows analogous information to thatshown in FIG. 1D for the improved 193 nm lasers of FIGS. 1A and 1B. Seethe detailed explanation above for FIG. 1D.

FIG. 2 shows a preferred embodiment of a fourth harmonic generator 200.The fundamental 201 at frequency ω is converted to the second harmonic(2ω) 202A by a first frequency doubling module 202. The second harmonic202A is converted to the fourth harmonic (4ω) 203A by a second frequencydoubling module 203. The unconsumed fundamental 202B and the unconsumedsecond harmonic 203B not used within the first frequency doubling module202 and the second frequency doubling module 203, respectively, may beoutput separately by these modules.

In one preferred embodiment of fourth harmonic generator 200, the secondharmonic generation module 202 may comprise a LBO crystal for frequencyconversion. In other embodiments, the second harmonic generation module202 may comprise a CLBO, BBO, or other non-linear crystal for frequencyconversion. In one preferred embodiment of fourth harmonic generator200, the fourth harmonic generation module 203 may comprise a CLBOcrystal for frequency conversion. In other embodiments, the fourthharmonic generation module 203 may comprise a BBO or other non-linearcrystal for frequency conversion.

FIG. 3 shows a preferred embodiment of a frequency mixer module 300. Infrequency mixer module 300, a first frequency mixer 302 receives afourth harmonic (4ω) 301A and a down-converted signal (ω_(s)) 301B togenerate a sum frequency 303A at a frequency equal to the sum of 4ω andω_(s). The sum frequency 303A and an unconsumed signal 303B at frequencyof ω_(s) are directed to a second frequency mixer 304, which generates alaser output 305 at a frequency of 4ω+2ω_(s). In some embodiments, thefirst frequency mixer 302 directs an unconsumed fourth harmonic 303Caway from the second frequency mixer 304. In other embodiments, theunconsumed fourth harmonic 303C can be passed to the second frequencymixer 304 because, in those embodiments, the polarization of the secondfrequency mixer 304 is oriented such that it has little or no effect onthe generation of the laser output 305.

In preferred embodiments, the sum frequency 303A is a frequency that isequivalent to a wavelength of approximately 224 nm. In some embodiments,the sum frequency 303A is substantially equal to the frequencycorresponding to a wavelength of 223.95 nm. In other embodiments, thesum frequency 303A is a frequency corresponding to a wavelength in therange from approximately 221 nm to approximately 229 nm. In yet otherembodiments, the sum frequency 303A is a frequency corresponding to awavelength in the range from approximately 222 nm to approximately 226nm. In preferred embodiments, frequency mixer 302 comprises a CLBOcrystal for frequency conversion. In preferred embodiments, frequencymixer 304 comprises a CLBO crystal for frequency conversion.

Note that the frequency mixing module 138 (FIG. 1C) may be implementedin a similar manner to that illustrated in FIG. 3 for frequency mixingmodule 300, except that its inputs are the third harmonic (3ω) and thesignal frequency (ω_(s)) instead of the fourth harmonic (4ω) and thesignal frequency (ω_(s)). Some appropriate crystals for use in thefrequency mixers 302 and 304 are described above. In particular, notethat when frequency mixing module 300 is used for frequency mixingmodule 138, CLBO cannot be used in frequency mixer 304 for generating awavelength near 193.4 nm because it does not phase match. As notedabove, BBO, or another non-linear crystal, must be used in frequencymixer 304 in this case.

Note that in any of the embodiments shown in FIGS. 1A, 1B, 1C, 2, and 3,mirrors may be used to direct the fundamental or other wavelengths asneeded. Prisms, beam splitters, beam combiners and dichroic coatedmirrors, for example, may be used to separate and combine beams asnecessary. Various combinations of mirrors and beam splitters may beused to separate and route the various wavelengths between differentfrequency generators and mixers in any appropriate sequence. The facesof frequency conversion crystals and prisms may be cut at an angleapproximately or substantially equal to Brewster's angle for an incidentwavelength in order to minimize or control reflection without using ananti-reflection coating. This cutting can be particularly advantageousfor those surfaces where UV radiation is incident, becauseanti-reflection coatings may degrade when exposed to UV and thus maydegrade the reliability of the laser if used on such surfaces.Waveplates or other optical elements may be used to rotate thepolarization of any of the wavelengths as needed to align thepolarization with the appropriate crystal axis of the next frequencyconversion or frequency mixing stage.

FIGS. 1A, 1B, and 1C illustrate various improved lasers for generating afrequency substantially equal to four times the fundamental frequency(three times the fundamental frequency for FIG. 1C) plus two times thesignal frequency, where the signal frequency is created bydown-conversion from the fundamental (down-conversion from the secondharmonic in FIG. 1C). Other laser embodiments similar to those describedabove for generating four times the fundamental frequency plus two timesthe signal frequency or three times the fundamental frequency plus twotimes the signal frequency are possible and are within the scope of thisinvention.

The above-described figures are not meant to represent the actualphysical layout of the components. The above-described figures show themain optical modules involved in the process, but do not show everyoptical element. One skilled in the appropriate arts would understandhow to build the improved laser from the above-described figures andtheir associated descriptions. It is to be understood that more or fewermirrors or prisms may be used to direct the light where needed. Lensesand/or curved mirrors may be used to focus the beam waist to foci ofsubstantially circular or elliptical cross sections inside or proximateto the non-linear crystals where appropriate. Prisms, gratings ordiffractive optical elements may be used to steer or separate thedifferent wavelengths at the outputs of each frequency convertor ormixer module when needed. Prisms, coated mirrors, or other elements maybe used to combine the different wavelengths at the inputs to thefrequency convertors and mixers as appropriate. Beam splitters or coatedmirrors may be used as appropriate to divide one wavelength into twobeams. Filters may be used to block undesired wavelengths at the outputof any stage. Waveplates may be used to rotate the polarization asneeded. Other optical elements may be used as appropriate. U.S. patentapplication Ser. No. 13/293,485, entitled “High Damage ThresholdFrequency Conversion System”, filed on May 17, 2012, and incorporated byreference herein, describes various optical elements particularly suitedto use in the frequency conversion stages that generate UV wavelengths.In some cases, it may be acceptable to allow unconsumed light from onefrequency conversion stage to pass to the next stage even though thatlight is not needed in the subsequent stage. This may be acceptable ifthe power density is low enough not to cause damage and if there islittle interference with the desired frequency conversion process (forexample because of no phase matching at the crystal angle or due to thepolarization of the light). One skilled in the appropriate arts wouldunderstand the various tradeoffs and alternatives that are possible inthe implementation of the improved laser.

In a preferred embodiment, the first frequency doubling module 110 (FIG.1B) that generates the second harmonic can include a Lithium triborate(LBO) crystal, which is substantially non-critically phase-matched attemperature of about 149° C. to produce light at approximately 532 nm.In another embodiment, the second frequency doubling module 112 (FIG.1B) that generates the fourth harmonic and the frequency mixers 302 and304 (FIG. 3) can use critical phase matching in Cesium Lithium Borate(CLBO), beta-Barium Borate (BBO), LBO, or other non-linear crystals. Inpreferred embodiments, the second frequency mixer 304 includes a CLBOcrystal that is critically phase matched with a high D_(eff)(approximately 0.5 to 1 pm/V) and a low walk-off angle (less than about35 mrad) at a temperature of approximately 100° C. This temperature is aconvenient temperature to use because it minimizes absorption of waterby the CLBO crystal. However, higher and lower temperatures may be usedwith appropriate adjustment of the angle of the crystal relative to theincident light. In some embodiments, the second frequency mixer 304includes BBO, which has a larger walk-off angle (approximately 100 mrad)and a larger D_(eff) (approximately 1 to 2 pm/V) compared with CLBO. Inpreferred embodiments, the frequency mixer 302 includes a CLBO crystalthat is critically phase matched with a high D_(eff) (approximately 0.8pm/V) and a low walk-off angle (less than about 40 mrad) at atemperature of approximately 100° C. In that case, the second frequencymixer 304 can include BBO, which has a larger walk-off angle(approximately 96 mrad) and a larger D_(eff) (approximately 1 to 2 pm/V)compared with CLBO.

The frequency doubling module 112 and the frequency mixers 302 and 304may advantageously use some, or all, of the methods and systemsdisclosed in co-pending U.S. patent application Ser. No. 13/412,564,filed on Mar. 5, 2012 and entitled “Laser with high quality, stableoutput beam, and long-life high-conversion-efficiency non-linearcrystal” by Dribinski et al. U.S. patent application Ser. No. 13/412,564claims priority from U.S. Provisional Application No. 61/510,633,entitled “Mode-locked UV laser with high quality, stable output beam,long-life high conversion efficiency non-linear crystal and a waferinspection system using a mode-locked laser”, which was filed on Jul.22, 2011. Both of these patent applications are incorporated byreference as if fully set forth herein.

Any of the harmonic generation stages (such as those performed byfrequency doubling modules 110 and 112) and frequency mixing stages(such as performed by frequency mixers 302 and 304) may include one ormore protective environments, such as those described in PCT PublishedPatent Application WO 2009/082460, entitled “Enclosure for controllingthe environment of optical crystals”, by J. Joseph Armstrong, andpublished on Jul. 2, 2009. This PCT publication is incorporated byreference as if fully set forth herein. Note that a single protectiveenvironment may enclose multiple stages or a single stage.

Further note that any of the harmonic generators (e.g. frequencydoubling modules 110 and 112) may advantageously use hydrogen-annealednon-linear crystals. Such crystals may be processed as described in U.S.Provisional Application 61/544,425 filed on Oct. 7, 2011 by Chuang etal. and in co-pending U.S. patent application Ser. No. 13/488,635 filedon Jun. 1, 2012 by Chuang et al. These applications are incorporated byreference as if fully set forth herein. The hydrogen annealed crystalsmay be particularly useful in those stages involving deep UVwavelengths, including the frequency doublers 112 and 203 and thefrequency mixers 302 and 304.

FIGS. 4A and 4B show some preferred embodiments of frequency mixers thatcan be used in the improved lasers described above. FIG. 4A illustratesone embodiment of a frequency mixer 400A in which type-II mixing is usedfor the first frequency mixing stage and type-I mixing is used for thefinal frequency mixing stage. Typically, three-wave mixing is done in abirefringent crystalline material (i.e. the refractive index depends onthe polarization and direction of the light that passes through), wherethe polarizations of the fields and the orientation of the crystal arechosen such that the phase-matching condition is achieved. Typically, abirefringent crystal has three axes, one or two of which have adifferent refractive index than the other one(s). For example, uniaxialcrystals have a single extraordinary (e) axis and two ordinary axes (o).If the two input frequencies have the same polarization (usuallyparallel to an o axis of the crystal), the phase matching is called“type-I phase-matching” and if their polarizations are perpendicular, itis called “type-II phase-matching”. However, other conventions existthat specify further which frequency has what polarization relative tothe crystal axis.

The first frequency mixing stage comprises a non-linear crystal 402,such as CLBO. The fourth harmonic and the signal wavelength enter thecrystal 402 approximately collinearly in direction 410 and are focusedto beam waists inside or proximate to this crystal (beam waists notshown). The signal wavelength is polarized with its electric-fieldvector in a direction shown by an arrow 420. The fourth harmonic ispolarized substantially perpendicularly to the signal wavelength. Theextraordinary (e) axis of the non-linear crystal 402 is orientedsubstantially parallel to the direction 420, as shown by an arrow 425.The plane containing the ordinary (o) axes of the non-linear crystal 402are oriented substantially perpendicular to the direction 420. The oaxes of the crystal 402 are rotated at an angle relative to thepropagation direction of the light in the crystal so as to achieve phasematching. For type-II matching in CLBO at a temperature of approximately100° C. with a signal wavelength near 1416 nm and a fourth harmonicwavelength near 266 nm, this angle is approximately 58.9°, and for BBOat approximately 100° C. with the same wavelengths, this angle isapproximately 45.7°. One skilled in the appropriate arts understands howto choose different combinations of temperature and angle to achievephase matching.

In some embodiments, the input surface 442 of crystal 402 is cut so asto be approximately at a Brewster's angle relative to the fourthharmonic (i.e. the direction 410). This angle minimizes reflection ofthe fourth harmonic wavelength without needing any anti-reflectioncoating on the input surface 442. In some embodiments, the input surface442 may have an anti-reflection coating to reduce the reflected light atthe fourth harmonic and/or the signal wavelengths. The output surface452 of the crystal 402 may be coated or uncoated. The advantage of notcoating is that coatings can have a short lifetime when exposed tointense UV radiation.

The output light 412 from crystal 402 is directed by optics 403 to thefinal frequency mixing stage, which includes a non-linear crystal 404.Note that three wavelengths exit crystal 402: the signal wavelength, thefourth harmonic wavelength, and a wavelength of the sum of the signaland the fourth harmonic. Because of the small walk-off angle, thesethree wavelengths are traveling approximately, but not exactly,collinearly. Optics 403 may include lenses, mirrors, prisms, beamsplitters, and/or other optical elements. The optics 403 may refocus thebeam waist inside, or proximate to, the crystal 404. In someembodiments, the optics 403 may approximately compensate for thewalk-off so as to make the wavelengths more collinear as they entercrystal 404. In some embodiments, the optics 403 may separate out anyunconsumed fourth harmonic. Other embodiments of the optics 403 may notseparate the unconsumed fourth harmonic because it is polarizedperpendicular to the polarization direction 421 and therefore will notbe phase matched in the crystal 404. The polarization direction 421 isthe direction of the electric field of both the signal wavelength andthe wavelength of the sum of the signal and fourth harmonic wavelength.The polarization direction 421 is substantially parallel to thepolarization direction 420 of the input signal wavelength. In someembodiments, the optics 403 may relatively delay by different amounts oftime the pulses at different wavelengths so that they arrivesubstantially simultaneously inside the crystal 404. In otherembodiments, the pulses are sufficiently long that the pulses overlapinside the crystal 404 without wavelength-dependent delays beingincorporated into the optics 403. In some embodiments, the optics 403may be omitted. The optics 403 may be omitted if the Rayleigh ranges ofall the wavelengths are long enough that efficient mixing occurs in bothcrystals 402 and 403 without refocusing, provided also that the walk-offis small enough relative to the beam diameters, and the pulse lengthsare long enough to have substantial overlap of the different wavelengthsinside both crystals. The optics 403 may be coated or uncoated.

The crystal 404 may have its input surface 444 oriented at approximatelya Brewster's angle for the wavelength corresponding to the sum of thesignal and fourth harmonic. In such embodiments, the surface 444 can beuncoated, which has the advantage of reducing the susceptibility todamage by high intensity UV radiation. In other embodiments, the surface444 may have an anti-reflection coating. In some embodiments, thesurface 444 may be substantially perpendicular to the output light 412.The crystal 404 is oriented so that the plane containing its o axes issubstantially parallel to the plane containing the polarizationdirection 421 and the direction of the output light 412 coming from thecrystal 402. The e axis of crystal 404 is substantially perpendicular tothe polarization direction 421. One o axis of the crystal 404 is rotatedto an angle relative to direction of the output light 412 so as toachieve phase matching. For type I matching in CLBO at approximately100° C. with wavelengths of approximately 1416 nm and 224 nm, this angleis approximately 65.4°. For BBO at approximately 100° C. for similarwavelengths, this angle is approximately 50.0°. In some embodiments, theoutput surface 454 is oriented at approximately a Brewster's anglerelative to the laser output wavelength so as to maximize thetransmission of the output wavelength. In the embodiment just described,the polarization direction of the output wavelength is perpendicular tothe polarization direction 421.

Preferred embodiments may use optics 405 to separate the desired outputwavelength, i.e. the laser output 450, from the other unwantedwavelengths 451. The optics 405 may include a beam splitter, a prism, agrating, or other optical elements. In some embodiments, the combinationof walk-off and the angle of the output surface 454 of the crystal 404may achieve sufficient separation of the laser output 450 from the otherwavelengths that the optics 405 are not required.

FIG. 4B illustrates an alternative embodiment of a frequency mixer 400Bin which type-I mixing is used for the first frequency mixing stage andtype II-mixing is used for the final frequency mixing stage. The firstfrequency mixing stage includes a non-linear crystal 402′, such as CLBO.The fourth harmonic (4ω) and the signal (ω_(s)) wavelength enter thecrystal 402′ approximately collinearly along a direction 410′ and arefocused to beam waists inside or proximate to the crystal 402′ (beamwaists not shown). In this embodiment, both the signal wavelength andthe fourth harmonic are polarized with their electric-field vectorssubstantially parallel to one another in the direction shown by thearrow 420′. The plane containing the o axes of the crystal 402′ isparallel to the plane containing the directions 420′ and the directionof propagation of the light in the crystal 402′. The e axis of thecrystal 402′ is perpendicular to the plane of the page of FIG. 4B. The oaxes of the crystal 402′ are rotated at an angle relative to thedirection of the light propagating in the crystal 402′ so as to achievephase matching. For type I matching in CLBO at a temperature ofapproximately 100° C. with a signal wavelength near 1416 nm and a fourthharmonic wavelength near 266 nm, this angle is approximately 53.5°. ForBBO at approximately 100° C. with similar wavelengths, this angle isapproximately 42.4°. One skilled in the appropriate arts understands howto choose different combinations of temperature and angle to achievephase matching.

In some embodiments, an input surface 442′ of the crystal 402′ is cut soas to be approximately at Brewster's angle relative to the direction410′ for the fourth harmonic wavelength. This angle minimizes reflectionof the fourth harmonic wavelength without needing any anti-reflectioncoating on the input surface 442′. In some embodiments, the inputsurface 442′ may have an anti-reflection coating to reduce the reflectedlight at the fourth harmonic and/or the signal wavelengths. An outputsurface 452′ of the crystal 402′ may be coated or uncoated. Theadvantage of not coating is that coatings can have a short lifetime whenexposed to intense UV radiation.

An output light 412′ from the crystal 402′ is directed by optics 403′ tothe final frequency mixing stage including a non-linear crystal 404′.The optics 403′ performs the same functions as the optics 403 describedabove in reference to FIG. 4A. The optics 403′ may be implemented withsimilar elements to those described for the optics 403. Some embodimentsof the frequency mixer 400B may omit the optics 403′.

A direction 421′ is the direction of the electric field of both thesignal and any unconsumed fourth harmonic. The direction 421′ issubstantially parallel to the direction 420′ of the signal wavelength.The light at the wavelength corresponding to the sum of the signal andfourth harmonic is polarized perpendicularly to the direction 421′.

In the frequency mixer 400B, a crystal 404′ may have its input surface444′ oriented at approximately Brewster's angle for the wavelengthcorresponding to the sum of the signal and fourth harmonic. In suchembodiments, the surface 444′ can be uncoated, which has the advantageof reducing the susceptibility to damage by high intensity UV radiation.In other embodiments, the surface 444′ may have an anti-reflectioncoating. In some embodiments, the surface 444′ may be substantiallyperpendicular to the light 412. The crystal 404′ is oriented so that theplane containing its o axes is substantially perpendicular to thedirection 421′, and its e axis is substantially parallel to thedirection 421′. The o axes of the crystal 404′ are rotated by an anglerelative to direction of propagation of the light inside the crystal404′ so as to achieve phase matching. For type-II matching in CLBO atapproximately 100° C. with wavelengths of approximately 1416 nm and 224nm, this angle is approximately 72.7°, and for BBO at approximately 100°C. for similar wavelengths, this angle is approximately 53.1°. In someembodiments, an output surface 454′ of the crystal 404′ is oriented atapproximately Brewster's angle relative to the laser output wavelengthso as to maximize the transmission of the output wavelength. In thefrequency mixer 400B, the polarization direction of the laser output450′ is perpendicular to the direction 421′.

Some preferred embodiments of the frequency mixer 400B use optics 405′to separate laser output 450′ from other unwanted wavelengths 451′. Theoptics 405′ may include a beam splitter, a prism, a grating, or otheroptical elements. In some embodiments, the combination of walk-off andthe angle of the output surface 454′ of crystal 404′ may achievesufficient separation of the laser output 450′ from the unwantedwavelengths 451′ that the optics 405′ are not required.

In preferred embodiments of the frequency mixers 400A and 400B, asubstantial fraction, or almost all, of the input fourth harmonic isconsumed in the crystal 402/402′ due to the use of a high power at thesignal wavelength. Some preferred embodiments use high enough powerlevels for the signal that a substantial fraction, or almost all, of thesum plus signal frequency created in the crystal 402/402′ is consumed inthe crystal 404/404′.

When CLBO crystals are used for the final two mixing stages (i.e. 402and 404 in frequency mixer 400A, or 402′ and 404′ in frequency mixer400B), the embodiment of FIG. 4A using type II mixing followed by type Imixing has the advantage of having approximately twice as largenon-linear coefficient for the final mixing (approximately 1 pm/Vcompared with approximately 0.5 pm/V), whereas the non-linearcoefficients are similar for the first mixing stage.

When BBO crystals are used for both first and final stages, thedifference in efficiency between the embodiments of FIGS. 4A and 4B issmall because type-I conversion is about approximately two times moreefficient than type-II for both mixing stages.

In some embodiments, in order to generate sufficient power at thefundamental wavelength of approximately 1064 nm, one or more amplifiersmay be used to increase the power of the fundamental. If two or moreamplifiers are used, then one seed laser should preferably be used toseed all the amplifiers so that they all output the same wavelength andthe laser pulses are synchronized. FIG. 5 illustrates an exemplaryamplifier module 500 in which a seed laser 503 can generate stabilized,narrow-band seed laser light 504 at the desired fundamental wavelength(e.g. approximately 1064 nm). In some embodiments, the seed laser 503 isone of a Nd doped YAG laser, a Nd-doped yttrium orthovanadate laser, afiber laser, or a stabilized diode laser. The seed light 504 goes to afirst amplifier 507 that amplifies the light to a higher power level. Insome embodiments, the first amplifier 507 comprises Nd-doped YAG orNd-doped yttrium orthovanadate. In one embodiment, an amplifier pump 505includes a laser that can pump the first amplifier 507. In someembodiments, this pumping can be done using one or more diode lasersoperating at approximately 808 nm in wavelength or at approximately 888nm in wavelength. In other embodiments, the first amplifier 507 maycomprise an Yb-doped fiber amplifier.

FIG. 5 also illustrates exemplary additional components that may be usedin some embodiments of the amplifier module 500. Because the OPO/OPA116, the frequency doubling module 110, and the frequency combiner 133(FIGS. 1A, 1B, 1C) receive the fundamental laser wavelength as an inputand depending on the output power required near 193.4 nm in wavelength,more fundamental laser light may be required that can be convenientlygenerated in a single amplifier at the required bandwidth, stability andbeam quality. Indeed, increasing the power output of an opticalamplifier can lead to increased bandwidth, degradation in the beamquality due to thermal lensing or other effects, reduced stability,and/or shortened lifetime.

Therefore, in some embodiments of the amplifier module 500, the firstamplifier 507 and an additional second amplifier 517 can be used torespectively generate two fundamental laser outputs 128 and 528, whichare directed to different frequency conversion stages. The secondamplifier 517 can be substantially identical to the first amplifier 507.In one embodiment, an amplifier pump 515 includes a laser that can pumpthe second amplifier 517. The amplifier pump 515 can be substantiallyidentical to the amplifier pump 505. Notably, the same seed laser 503can be used to seed both lasers in order to ensure that the outputs 128and 528 are at the same wavelength and are synchronized. A beam splitter511 and a mirror 512 can divide the seed light 504 and direct a fractionof it to the second amplifier 517.

FIG. 6 illustrates an exemplary OPO/OPA 600 configured to generate thedown-converted signal at frequency ω_(s). In this embodiment, a beamcombiner 611 combines an input laser light 600 at the fundamentalwavelength (the wavelength corresponding to the fundamental frequency ω)and seed laser light 604 at a wavelength corresponding to thedown-converted signal frequency ω_(s), which is generated by a seedlaser 603.

The beam combiner 611 may comprise a dichroic coating that efficientlyreflects a first wavelength while transmitting a second wavelength (forexample, the beam combiner 611 may reflect the fundamental and transmitthe seed laser light as shown, or vice-versa, not shown). After the beamcombiner, the first and second wavelengths travel substantiallycollinearly through a non-linear converter 607, which may compriseperiodically poled lithium niobate, magnesium oxide doped lithiumniobate, KTP, or another suitable non-linear crystalline material.

In some preferred embodiments, the seed laser 603, such as a diode laseror a low-powered fiber laser, generates the seed laser light 604 havinga wavelength of approximately 1416 nm (such as within a range from about1330 nm to about 1612 nm, or within a range from about 1378 nm to 1461nm, or within a range from about 1413 nm to about 1417 nm), which isthen used to seed the down conversion process at the design frequencyω_(s). The seed laser 603 need only be of approximately 1 mW, a few mW,or a few tens of mW in power. In some preferred embodiments, the seedlaser 603 is stabilized by using, for example, a grating and stabilizingthe temperature. The seed laser 603 should preferably generate polarizedlight, which is then introduced into the non-linear converter 607polarized substantially perpendicular to the polarization of thefundamental, i.e. the input laser light 602. In some embodiments, anon-linear crystal may be contained in a resonant cavity of thenon-linear converter 607 to create a laser/amplifier based onspontaneous emission.

In this embodiment, a beam splitter 621 (or a prism in otherembodiments) can separate the down-converted signal 129 at wavelengthω_(s) from an unconsumed fundamental 623. In other embodiments (notshown), the unconsumed fundamental 623 may be recirculated back to theinput of the non-linear converter 607 with a time delay to match thenext incoming laser pulse of the input laser light 602. Note that theOPO/OPA 136 (FIG. 1C) can be implemented in a similar configuration tothat shown in FIG. 6 except that the input laser light 602 is at thesecond harmonic (2ω), rather than the fundamental (ω).

A quasi-CW laser may be constructed using a high repetition rate laser,such as a mode-locked laser operating at approximately 50 MHz or ahigher repetition rate for the fundamental laser 102. A true CW lasermay be constructed using a CW laser for the fundamental laser 102. A CWlaser may need one or more of the frequency conversion stages to becontained in resonant cavities to build up sufficient power density toget efficient frequency conversion.

In one embodiment, a fundamental wavelength near 1030 nm is used insteadof a fundamental of approximately 1064 nm. Such a wavelength can begenerated by a laser using a gain medium of a Yb-doped YAG crystal or aYb-doped fiber. With a 1030 nm fundamental wavelength, an OPO signalwavelength of approximately 1552.8 nm will generate a final outputwavelength near 193.368 nm. A substantially similar frequency conversionscheme to the above can be used to generate the second harmonic (near515 nm), the fourth harmonic (near 257.5 nm), the sum of the signal(ω_(s)) and the fourth harmonic (near 220.9 nm), and the final outputwavelength. BBO or CLBO may be used for the UV frequency conversion andmixing stages. Other non-linear crystals may also be suitable for someof the frequency conversion or mixing stages.

In yet another embodiment, a fundamental wavelength near 1047 nm or near1053 nm is used instead of a fundamental of approximately 1064 nm. Alaser operating near 1047 nm or near 1053 nm may be based on Nd:YLF(neodymium-doped yttrium lithium fluoride) for example. An appropriatesignal wavelength can be chosen so as to achieve the desired laseroutput wavelength. For example, a signal frequency ω_(s) having awavelength of approximately 1480 nm can generate a laser output of4ω+2ω_(s) near 193.4 nm from a fundamental near 1047 nm. Alternatively,a signal frequency corresponding to a wavelength of approximately 1457nm could be used with a fundamental near 1053 nm to generate a similarlaser output.

In yet other embodiments, fundamental laser wavelengths near 1030 nm,1047 nm, or 1053 nm can generate an output wavelength of approximately193.4 nm using 3ω+2ω_(s), where ω_(s) is approximately 885 nm, 867 nm,or 861 nm, respectively.

The improved 193 nm laser is less complex and more efficient than, forexample, 8th harmonic generation (which generally needs more frequencyconversion stages), and much less complex than combining two differentfundamental wavelengths. Therefore, the above-described improved 193 nmlaser can provide significant system advantages during photomask,reticle, or wafer inspection.

The improved 193 nm laser has several advantages compared with the laserof U.S. patent application Ser. No. 13/797,939, filed by Chuang et al.on Mar. 12, 2013 (P3913). A first advantage is that the final frequencyconversion stage of the improved 193 nm laser is more efficient atgenerating higher output power for a given fundamental power, or a lowerpower fundamental for the same output power. A second advantage is thatoptical components and test equipment operating near 1.4 μm or 1.5 μmare more readily available than near 2.1 μm in wavelength. A thirdadvantage is that for a signal wavelength near 1.4 μm or 1.5 μmsignificantly more energy goes into the signal compared with the idler,thereby resulting in more efficient conversion of fundamental power tooutput power (compared with a signal wavelength near 2.1 μm where almostequal amounts of power must go into the signal and the idler). A fourthadvantage is that a signal wavelength near 1.4 μm or near 1.5 μm is notclose to any water or —OH absorption peak, therefore leading to greatertolerance of small amounts of moisture in any of the crystals or in thelight path. A sixth advantage is that the final frequency mixing stage(e.g. frequency mixing module 103, FIG. 1A) uses only two inputwavelengths (i.e. the fourth harmonic and the signal) instead of threeinput wavelengths (i.e. the fourth harmonic, the fundamental, and thesignal). Note that many of the same advantages apply to the improved 193nm laser (FIG. 1C) that combines the third harmonic with a signalwavelength between about 800 nm and 900 nm in wavelength.

Fundamental lasers operating near 1064 nm, 1053 nm, 1047 nm, and 1030 nmin wavelength are readily available at a range of different power levelsand repetition rates including mode-locked, Q-switched, quasi-CW, andtrue CW lasers. The improved 193 nm laser is capable of operating atrepetitions rates higher than 1 MHz, which is important for high-speedinspection applications. The use of mode-locked or quasi-CW fundamentallaser operating at a repetition rate greater than 50 MHz, isparticularly advantageous for high-speed inspection of semiconductorwafers, photomasks, and reticles because it allows high-speed imageacquisition and reduces the peak power of each pulse (and so causes lessdamage to the optics and to the article being inspected) compared with alower repetition rate laser of the same power. Although the aboveembodiments describe using various fundamental wavelengths to generate alaser output of 193.3 nm, other wavelengths within a few nanometers of193.3 nm can be generated using an appropriate choice of signalwavelength. Such lasers and systems utilizing such lasers are within thescope of this invention.

FIGS. 7-14 illustrate systems that can include one of theabove-described improved 193 nm lasers. These systems can be used inphotomask, reticle, or wafer inspection and metrology applications.

FIG. 7 shows a reticle, photomask, or wafer inspection system 700 thatsimultaneously detects two channels of image or signal on one sensor770. The illumination source 709 incorporates an improved 193 nm laseras described herein. The light source may further comprise a pulsemultiplier and/or a coherence reducing scheme. The two channels maycomprise reflected and transmitted intensity when an inspected object730 is transparent (for example a reticle or photomask), or may comprisetwo different illumination modes, such as angles of incidence,polarization states, wavelength ranges or some combination thereof.

As shown in FIG. 7, illumination relay optics 715 and 720 relay theillumination from source 709 to the inspected object 730. The inspectedobject 730 may be a reticle, a photomask, a semiconductor wafer or otherarticle to be inspected. Image relay optics 740, 755, and 760 relay thelight that is reflected and/or transmitted by the inspected object 730to the sensor 770. The data corresponding to the detected signals orimages for the two channels is shown as data 780 and is transmitted to acomputer (not shown) for processing.

Other details of an embodiment of a reticle or photomask inspectionsystem that may be configured to measure transmitted and reflected lightfrom the reticle or photomask are described in U.S. Pat. Nos. 7,352,457and 7,528,943, which are incorporated by reference herein. Additionaldetails on reticle and photomask inspection systems that may incorporatethe improved 193 nm laser are provided by U.S. Pat. Nos. 7,528,943 and5,563,702, both of which are incorporated by reference herein.

FIG. 8 illustrates an exemplary inspection system 800 including multipleobjectives and one of the above-described improved 193 nm lasers. Insystem 800, illumination from a laser source 801 is sent to multiplesections of the illumination subsystem. A first section of theillumination subsystem includes elements 802 a through 806 a. Lens 802 afocuses light from laser source 801. Light from lens 802 a then reflectsfrom mirror 803 a. Mirror 803 a is placed at this location for thepurposes of illustration, and may be positioned elsewhere. Light frommirror 803 a is then collected by lens 804 a, which forms illuminationpupil plane 805 a. An aperture, filter, or other device to modify thelight may be placed in pupil plane 805 a depending on the requirementsof the inspection mode. Light from pupil plane 805 a then passes throughlens 806 a and forms illumination field plane 807.

A second section of the illumination subsystem includes elements 802 bthrough 806 b. Lens 802 b focuses light from laser source 801. Lightfrom lens 802 b then reflects from mirror 803 b. Light from mirror 803 bis then collected by lens 804 b which forms illumination pupil plane 805b. An aperture, filter, or other device to modify the light may beplaced in pupil plane 805 b depending on the requirements of theinspection mode. Light from pupil plane 805 b then passes through lens806 b and forms illumination field plane 807. The light from the secondsection is then redirected by mirror or reflective surface such that theillumination field light energy at illumination field plane 807 iscomprised of the combined illumination sections.

Field plane light is then collected by lens 809 before reflecting off abeamsplitter 810. Lenses 806 a and 809 form an image of firstillumination pupil plane 805 a at objective pupil plane 811. Likewise,lenses 806 b and 809 form an image of second illumination pupil plane805 b at objective pupil plane 811. An objective 812 (or alternatively813) then takes the pupil light and forms an image of illumination field807 at sample 814. Objective 812 or objective 813 can be positioned inproximity to sample 814. Sample 814 can move on a stage (not shown),which positions the sample in the desired location. Light reflected andscattered from the sample 814 is collected by the high NA catadioptricobjective 812 or objective 813. After forming a reflected light pupil atobjective pupil plane 811, light energy passes through beamsplitter 810and lens 815 before forming an internal field 816 in the imagingsubsystem. This internal imaging field is an image of sample 814 andcorrespondingly illumination field 807. This field may be spatiallyseparated into multiple fields corresponding to the illumination fields.Each of these fields can support a separate imaging mode. For example,one imaging mode may be a bright-field imaging mode, while another maybe a dark-field imaging mode.

One of these fields can be redirected using mirror 817. The redirectedlight then passes through lens 818 b before forming another imagingpupil 819 b. This imaging pupil is an image of pupil 811 andcorrespondingly illumination pupil 805 b. An aperture, filter, or otherdevice to modify the light may be placed in pupil plane 819 b dependingon the requirements of the inspection mode. Light from pupil plane 819 bthen passes through lens 820 b and forms an image on sensor 821 b. In asimilar manner, light passing by mirror or reflective surface 817 iscollected by lens 818 a and forms imaging pupil 819 a. Light fromimaging pupil 819 a is then collected by lens 820 a before forming animage on detector 821 a. Light imaged on detector 821 a can be used fora different imaging mode from the light imaged on sensor 821 b.

The illumination subsystem employed in system 800 is composed of lasersource 801, collection optics 802-804, beam shaping components placed inproximity to a pupil plane 805, and relay optics 806 and 809. Aninternal field plane 807 is located between lenses 806 and 809. In onepreferred configuration, laser source 801 can include one of theabove-described improved 193 nm lasers.

With respect to laser source 801, while illustrated as a single uniformblock having two points or angles of transmission, in reality thisrepresents a laser source able to provide two channels of illumination,for example a first channel of light energy such as laser light energyat a first frequency (e.g. a deep UV wavelength near 193 nm) whichpasses through elements 802 a-806 a, and a second channel of lightenergy such as laser light energy at a second frequency (e.g. adifferent harmonic, such as the 4^(th) harmonic, from the same laser, ora light from a different laser) which passes through elements 802 b-806b.

While light energy from laser source 801 is shown to be emitted 90degrees apart, and the elements 802 a-806 a and 802 b-806 b are orientedat 90 degree angles, in reality light may be emitted at variousorientations, not necessarily in two dimensions, and the components maybe oriented differently than as shown. FIG. 8 is therefore simply arepresentation of the components employed and the angles or distancesshown are not to scale nor specifically required for the design.

Elements placed in proximity to pupil plane 805 a/805 b may be employedin the current system using the concept of aperture shaping. Using thisdesign, uniform illumination or near uniform illumination may berealized, as well as individual point illumination, ring illumination,quadrapole illumination, or other desirable patterns.

Various implementations for the objectives may be employed in a generalimaging subsystem. A single fixed objective may be used. The singleobjective may support all the desired imaging and inspection modes. Sucha design is achievable if the imaging system supports a relatively largefield size and relatively high numerical aperture. Numerical aperturecan be reduced to a desired value by using internal apertures placed atthe pupil planes 805 a, 805 b, 819 a, and 819 b.

Multiple objectives may also be used as shown in FIG. 8. For example,although two objectives 812 and 813 are shown, any number is possible.Each objective in such a design may be optimized for each wavelengthproduced by laser source 801. These objectives 812 and 813 can eitherhave fixed positions or be moved into position in proximity to thesample 814. To move multiple objectives in proximity to the sample,rotary turrets may be used as are common on standard microscopes. Otherdesigns for moving objectives in proximity of a sample are available,including but not limited to translating the objectives laterally on astage, and translating the objectives on an arc using a goniometer. Inaddition, any combination of fixed objectives and multiple objectives ona turret can be achieved in accordance with the present system.

The maximum numerical apertures of this configuration may approach orexceed 0.97, but may in certain instances be smaller. The wide range ofillumination and collection angles possible with this high NAcatadioptric imaging system, combined with its large field size allowsthe system to simultaneously support multiple inspection modes. As maybe appreciated from the previous paragraphs, multiple imaging modes canbe implemented using a single optical system or machine in connectionwith the illumination device. The high NA disclosed for illumination andcollection permits the implementation of imaging modes using the sameoptical system, thereby allowing optimization of imaging for differenttypes of defects or samples.

The imaging subsystem also includes intermediate image forming optics815. The purpose of the image forming optics 815 is to form an internalimage 816 of sample 814. At this internal image 816, a mirror 817 can beplaced to redirect light corresponding to one of the inspection modes.It is possible to redirect the light at this location because the lightfor the imaging modes are spatially separate. The image forming optics818 (818 a and 818 b) and 820 (820 a and 820 b) can be implemented inseveral different forms including a varifocal zoom, multiple afocal tubelenses with focusing optics, or multiple image forming mag tubes. U.S.Published Application 2009/0180176, which published on Jul. 16, 2009 andis incorporated by reference herein, describes additional detailsregarding system 800.

FIG. 9 illustrates the addition of a normal incidence laser dark-fieldillumination to a catadioptric imaging system 900. The dark-fieldillumination includes a laser 901, adaptation optics 902 to control theillumination beam size and profile on the surface being inspected, anaperture and window 903 in a mechanical housing 904, and a prism 905 toredirect the laser along the optical axis at normal incidence to thesurface of a sample 908. Prism 905 also directs the specular reflectionfrom surface features of sample 908 and reflections from the opticalsurfaces of an objective 906 along the optical path to an image plane909. Lenses for objective 906 can be provided in the general form of acatadioptric objective, a focusing lens group, and a zooming tube lenssection. In a preferred embodiment, laser 901 can be implemented by theone of the above-described improved 193 nm lasers. Published US PatentApplication 2007/0002465, which published on Jan. 4, 2007 and isincorporated by reference herein, describes system 900 in furtherdetail.

FIG. 10A illustrates a surface inspection apparatus 1000 that includesillumination system 1001 and collection system 1010 for inspecting areasof surface 1011. As shown in FIG. 10A, a laser system 1020 directs alight beam 1002 through a lens 1003. In a preferred embodiment, lasersystem 1020 includes one of the above-described improved 193 nm lasers,an annealed crystal, and a housing to maintain the annealed condition ofthe crystal during standard operation by protecting it from moisture orother environmental contaminants. First beam shaping optics can beconfigured to receive a beam from the laser and focus the beam to anelliptical cross section at a beam waist in or proximate to the crystal.

Lens 1003 is oriented so that its principal plane is substantiallyparallel to a sample surface 1011 and, as a result, illumination line1005 is formed on surface 1011 in the focal plane of lens 1003. Inaddition, light beam 1002 and focused beam 1004 are directed at anon-orthogonal angle of incidence to surface 1011. In particular, lightbeam 1002 and focused beam 1004 may be directed at an angle betweenabout 1 degree and about 85 degrees from a normal direction to surface1011. In this manner, illumination line 1005 is substantially in theplane of incidence of focused beam 1004.

Collection system 1010 includes lens 1012 for collecting light scatteredfrom illumination line 1005 and lens 1013 for focusing the light comingout of lens 1012 onto a device, such as charge coupled device (CCD)1014, comprising an array of light sensitive detectors. In oneembodiment, CCD 1014 may include a linear array of detectors. In suchcases, the linear array of detectors within CCD 1014 can be orientedparallel to illumination line 1015. In one embodiment, multiplecollection systems can be included, wherein each of the collectionsystems includes similar components, but differ in orientation.

For example, FIG. 10B illustrates an exemplary array of collectionsystems 1031, 1032, and 1033 for a surface inspection apparatus (whereinits illumination system, e.g. similar to that of illumination system1001, is not shown for simplicity). First optics in collection system1031 collect light scattered in a first direction from the surface ofsample 1011. Second optics in collection system 1032 collect lightscattered in a second direction from the surface of sample 1011. Thirdoptics in collection system 1033 collect light scattered in a thirddirection from the surface of sample 1011. Note that the first, second,and third paths are at different angles of incidence to said surface ofsample 1011. A platform 1035 supporting sample 1011 can be used to causerelative motion between the optics and sample 1011 so that the wholesurface of sample 1011 can be scanned. U.S. Pat. No. 7,525,649, whichissued on Apr. 28, 2009 and is incorporated by reference herein,describes surface inspection apparatus 1000 and other multiplecollection systems in further detail.

FIG. 11 illustrates a surface inspection system 1100 that can be usedfor inspecting anomalies on a surface 1101. In this embodiment, surface1101 can be illuminated by a substantially stationary illuminationdevice portion of a laser system 1130 comprising one of theabove-described improved 193 nm lasers. The output of laser system 1130can be consecutively passed through polarizing optics 1121, a beamexpander and aperture 1122, and beam-forming optics 1123 to expand andfocus the beam.

The focused laser beam 1102 is then reflected by a beam foldingcomponent 1103 and a beam deflector 1104 to direct the beam 1105 towardssurface 1101 for illuminating the surface. In the preferred embodiment,beam 1105 is substantially normal or perpendicular to surface 1101,although in other embodiments beam 1105 may be at an oblique angle tosurface 1101.

In one embodiment, beam 1105 is substantially perpendicular or normal tosurface 1101 and beam deflector 1104 reflects the specular reflection ofthe beam from surface 1101 towards beam turning component 1103, therebyacting as a shield to prevent the specular reflection from reaching thedetectors. The direction of the specular reflection is along line SR,which is normal to the surface 1101 of the sample. In one embodimentwhere beam 1105 is normal to surface 1101, this line SR coincides withthe direction of illuminating beam 1105, where this common referenceline or direction is referred to herein as the axis of inspection system1100. Where beam 1105 is at an oblique angle to surface 1101, thedirection of specular reflection SR would not coincide with the incomingdirection of beam 1105; in such instance, the line SR indicating thedirection of the surface normal is referred to as the principal axis ofthe collection portion of inspection system 1100.

Light scattered by small particles are collected by mirror 1106 anddirected towards aperture 1107 and detector 1108. Light scattered bylarge particles are collected by lenses 1109 and directed towardsaperture 1110 and detector 1111. Note that some large particles willscatter light that is also collected and directed to detector 1107, andsimilarly some small particles will scatter light that is also collectedand directed to detector 1111, but such light is of relatively lowintensity compared to the intensity of scattered light the respectivedetector is designed to detect. In one embodiment, detector 1111 caninclude an array of light sensitive elements, wherein each lightsensitive element of the array of light sensitive elements is configuredto detect a corresponding portion of a magnified image of theillumination line. In one embodiment, inspection system can beconfigured for use in detecting defects on unpatterned wafers. U.S. Pat.No. 6,271,916, which issued to Marx et al. on Aug. 7, 2001 and isincorporated by reference herein, describes inspection system 1100 infurther detail.

FIG. 12 illustrates an inspection system 1200 configured to implementanomaly detection using both normal and oblique illumination beams. Inthis configuration, a laser system 1230, which includes one of theabove-described improved 193 nm lasers, can provide a laser beam 1201. Alens 1202 focuses the beam 1201 through a spatial filter 1203 and lens1204 collimates the beam and conveys it to a polarizing beam splitter1205. Beam splitter 1205 passes a first polarized component to thenormal illumination channel and a second polarized component to theoblique illumination channel, where the first and second components areorthogonal. In the normal illumination channel 1206, the first polarizedcomponent is focused by optics 1207 and reflected by mirror 1208 towardsa surface of a sample 1209. The radiation scattered by sample 1209 iscollected and focused by a paraboloidal mirror 1210 to a detector orphotomultiplier tube 1211.

In the oblique illumination channel 1212, the second polarized componentis reflected by beam splitter 1205 to a mirror 1213 which reflects suchbeam through a half-wave plate 1214 and focused by optics 1215 to sample1209. Radiation originating from the oblique illumination beam in theoblique channel 1212 and scattered by sample 1209 is collected byparaboloidal mirror 1210 and focused to photomultiplier tube 1211.Detector or photomultiplier tube 1211 has a pinhole entrance. Thepinhole and the illuminated spot (from the normal and obliqueillumination channels on surface 1209) are preferably at the foci of theparaboloidal mirror 1210.

The paraboloidal mirror 1210 collimates the scattered radiation fromsample 1209 into a collimated beam 1216. Collimated beam 1216 is thenfocused by an objective 1217 and through an analyzer 1218 to thephotomultiplier tube 1211. Note that curved mirrored surfaces havingshapes other than paraboloidal shapes may also be used. An instrument1220 can provide relative motion between the beams and sample 1209 sothat spots are scanned across the surface of sample 1209. U.S. Pat. No.6,201,601, which issued to Vaez-Iravani et al. on Mar. 13, 2001 and isincorporated by reference herein, describes inspection system 1200 infurther detail.

FIG. 13 illustrates an exemplary pulse multiplier 1300 for use with theabove-described improved 193 nm laser in an inspection or metrologysystem. Pulse multiplier 1300 is configured to generate pulse trainsfrom each input pulse 1301 from improved 193 nm laser 1310. Input pulse1301 impinges on a polarizing beam splitter 1302, which because of thepolarization of input pulse 1301, transmits all of its light to a lens1306. Thus, the transmitted polarization is parallel to the inputpolarization of input pulse 1301. Lens 1306 focuses and directs thelight of input pulse 1301 to a half-wave plate 1305. In general, a waveplate can shift the phases between perpendicular polarization componentsof a light wave. For example, a half-wave plate receiving linearlypolarized light can generate two waves, one wave parallel to the opticalaxis and another wave perpendicular to the optical axis. In half-waveplate 1305, the parallel wave can propagate slightly slower than theperpendicular wave. Half-wave plate 105 is fabricated such that forlight exiting, one wave is exactly half of a wavelength delayed (180degrees) relative to the other wave.

Thus, half-wave plate 1305 can generate pulse trains from each inputpulse 1301. The normalized amplitudes of the pulse trains are: cos 2θ(wherein θ is the angle of half-wave plate 1305), sin² 2θ, sin² 2θ cos2θ, sin² 2θ cos² 2θ, sin² 2θ cos³ 2θ, sin² 2θ cos⁴ 2θ, sin² 2θ cos⁵ 2θ,etc. Notably, the total energy of the pulse trains from a laser pulsecan be substantially conserved traversing half-wave plate 1305.

The sum of the energy from the odd terms generated by half-wave plate1305 is equal to:

(cos  2θ)² + (sin²2θcos 2θ)² + (sin²2θcos³2θ)² + (sin²2θcos⁵2θ)² + (sin²2θcos⁷2θ)2 + (sin²2θcos⁹2θ)² + …  = cos²2θ + sin⁴2θ(cos²2θ + cos⁶2θ + cos¹⁰2θ + …) = 2cos²2θ/(1 + cos²2θ)

In contrast, the sum of the energy from the even terms generated byhalf-wave plate 1305 is equal to:

(sin² 2θ)² + (sin²2θcos²2θ)² + (sin²2θcos⁴2θ)² + (sin²2θcos⁶2θ)² + (sin²2θcos⁸2θ)² + (sin²2θcos¹⁰2θ)² + …  = sin⁴2θ(1 + cos⁴2θ + cos⁸2θ + cos¹²2θ + …) = sin²2θ/(1 + cos²2θ)

In accordance with one aspect of pulse multiplier 1300, the angle θ ofhalf-wave plate 1305 can be determined (as shown below) to provide thatthe odd term sum is equal to the even term sum.2 cos² 2θ=sin² 2θcos² 2θ=⅓sin² 2θ=⅔θ=27.4 degrees

The light exiting half-wave plate 1305 is reflected by mirrors 1304 and1303 back to polarizing beam splitter 1302. Thus, polarizing beamsplitter 1302, lens 1306, half-wave plate 1305, and mirrors 1304 and1303 form a ring cavity configuration. The light impinging on polarizingbeam splitter 1302 after traversing the ring cavity has twopolarizations as generated by half-wave plate 1305. Therefore,polarizing beam splitter 1302 transmits some light and reflects otherlight, as indicated by arrows 1309. Specifically, polarizing beamsplitter 1302 transmits the light from mirror 1303 having the samepolarization as input pulse 1301. This transmitted light exits pulsemultiplier 1300 as output pulses 1307. The reflected light, which has apolarization perpendicular to that of input pulse 1301, is re-introducedinto the ring cavity (pulses not shown for simplicity).

Notably, these re-introduced pulses can traverse the ring in the mannerdescribed above with further partial polarization switching by half-waveplate 1305 and then light splitting by polarizing beam splitter 1302.Thus, in general, the above-described ring cavity is configured to allowsome light to exit and the rest of the light (with some minimal losses)to continue around the ring. During each traversal of the ring (andwithout the introduction of additional input pulses), the energy of thetotal light decreases due to the light exiting the ring as output pulses1307.

Periodically, a new input pulse 1301 is provided by laser 1310 to pulsemultiplier 1300. In one embodiment, the laser may generate approximately0.1 nanosecond (ns) laser pulses at a repetition rate of approximately125 MHz, and the cavity may double the repetition rate. Note that thesize of the ring, and thus the time delay of the ring, can be adjustedby moving mirror 1304 along the axis indicated by arrows 1308.

The ring cavity length may be slightly greater than, or slightly lessthan, the nominal length calculated directly from the pulse intervaldivided by the multiplication factor. This results in the pulses notarriving at exactly the same time as the polarized beam splitter andslightly broadens the output pulse. For example, when the input pulserepetition rate is 125 MHz, the cavity delay would nominally be 4 ns fora frequency multiplication by 2. In one embodiment, a cavity lengthcorresponding to 4.05 ns can be used so that the multiply reflectedpulses do not arrive at exactly the same time as an incoming pulse.Moreover, the 4.05 ns cavity length for the 125 MHz input pulserepetition rate can also advantageously broaden the pulse and reducepulse height. Other pulse multipliers having different input pulse ratescan have different cavity delays.

Notably, polarizing beam splitter 1302 and half-wave plate 1305 workingin combination generate even and odd pulses, which diminish for eachround traversed inside the ring. These even and odd pulses can becharacterized as providing energy envelopes, wherein an energy envelopeconsists of an even pulse train (i.e. a plurality of even pulses) or anodd pulse train (i.e. a plurality of odd pulses). In accordance with oneaspect of pulse multiplier 1300, these energy envelopes aresubstantially equal.

More details of pulse multiplication can be found in co-pending U.S.patent application Ser. No. 13/487,075, entitled “SemiconductorInspection And Metrology System Using Laser Pulse Multiplier” and filedon Jun. 1, 2012, which is incorporated by reference herein. Note thatthe above pulse multiplier is just one example that may be used with theimproved 193 nm laser. Combining this improved laser with other pulsemultipliers is within the scope of this invention. For example, theimproved 193 nm laser described herein may also be used with any of thelaser pulse multipliers described in U.S. patent application Ser. No.13/711,593, entitled “Semiconductor Inspection and Metrology SystemUsing Laser Pulse Multiplier”, filed by Chuang et al. on Dec. 11, 2012,and incorporated by reference herein.

FIG. 14 illustrates aspects of a pulse-shaping or coherence reducingdevice used in conjunction with a pulsed laser, suitable forincorporation into an inspection or metrology system in accordance withembodiments of the present invention. A light source 1410 comprises animproved 193 nm laser as described herein. The light source 1410generates a light beam 1412 comprising a series of pulses. One aspect ofthis embodiment is to make use of the finite spectral range of the laserin order to perform a substantially quick temporal modulation of a lightbeam 1412, which can be changed on the required tenth picosecond timeintervals (a tenth picoseconds time interval is equivalent to a few nmin spectral width), and transform the temporal modulation to spatialmodulation.

The use of a dispersive element and an electro-optic modulator isprovided for speckle reduction and/or pulse shaping. For example, theillumination subsystem includes a dispersive element positioned in thepath of the coherent pulses of light. As shown in FIG. 14, thedispersive element can be positioned at a plane 1414 arranged at angleθ₁ to the cross-section of the coherent pulses of light. As shown inFIG. 14, the pulses of light exit the dispersive element at angle θ₁′and with cross-sectional dimension x₁′. In one embodiment, thedispersive element is a prism. In another embodiment, the dispersiveelement is a diffraction grating. The dispersive element is configuredto reduce coherence of the pulses of light by mixing spatial andtemporal characteristics of light distribution in the pulses of light.In particular, a dispersive element such as a prism or diffractiongrating provides some mixing between spatial and temporalcharacteristics of the light distribution in the pulses of light. Thedispersive element may include any suitable prism or diffractiongrating, which may vary depending on the optical characteristics of theillumination subsystem and the metrology or inspection system.

The illumination subsystem further includes an electro-optic modulatorpositioned in the path of the pulses of light exiting the dispersiveelement. For example, as shown in FIG. 14, the illumination subsystemmay include an electro-optic modulator 1416 positioned in the path ofthe pulses of light exiting the dispersive element. The electro-opticmodulator 1416 is configured to reduce the coherence of the pulses oflight by temporally modulating the light distribution in the pulses oflight. In particular, the electro-optic modulator 1416 provides anarbitrary temporal modulation of the light distribution. Therefore, thedispersive element and the electro-optic modulator 1416 have a combinedeffect on the pulses of light generated by the light source. Inparticular, the combination of the dispersive element with theelectro-optic modulator 1416 creates an arbitrary temporal modulationand transforms the temporal modulation to an arbitrary spatialmodulation of the output beam 1418.

In one embodiment, the electro-optic modulator 1416 is configured tochange the temporal modulation of the light distribution in the pulsesof light at tenth picosecond time intervals. In another embodiment, theelectro-optic modulator 1416 is configured to provide about 10³aperiodic samples on each period thereby providing a de-coherence timeof about 10⁻¹³ seconds.

Further details of the pulse-shaping, coherence, and speckle reducingapparatus and methods are disclosed in co-pending U.S. Published PatentApplication 2011/0279819, published on Nov. 17, 2011, and U.S. PublishedPatent Application 2011/0228263, published on Sep. 22, 2011. Both ofthese applications are incorporated by reference herein.

The various embodiments of the structures and methods described hereinare illustrative only of the principles of the invention and are notintended to limit the scope of the invention to the particularembodiments described. For example, non-linear crystals other than CLBO,LBO, or BBO or periodically-poled materials can be used for some of thefrequency conversion and mixing stages.

The invention claimed is:
 1. A laser for generating an output wavelengthof approximately 193.4 nm, the laser comprising: a fundamental laser; afirst frequency doubling module, coupled to the fundamental laser, forgenerating a second harmonic; a second frequency doubling module,coupled to the first frequency doubling module, for generating a fourthharmonic; an optical parametric generator, coupled to one of the firstfrequency doubling module and the fundamental laser, for generating adown-converted signal; and a frequency mixing module, coupled to theoptical parametric generator and the second frequency doubling module,for generating a laser output of a frequency equal to a sum of thefourth harmonic and twice a frequency of the down-converted signal,wherein the frequency mixing module comprises two non-linear crystals, afirst non-linear crystal configured to generate a frequency equal to asum of the fourth harmonic and a frequency of the down-converted signalby type-I conversion, and a second non-linear crystal configured togenerate the frequency equal to the sum of the fourth harmonic and thetwice the frequency of the down-converted signal by type-II conversion,and wherein the fundamental laser generates a fundamental frequency ofapproximately 1064.3 nm, approximately 1053 nm, approximately 1047 nm,or approximately 1030 nm.
 2. The laser of claim 1, wherein the opticalparametric generator includes a periodically poled non-linear opticalcrystal.
 3. The laser of claim 2, wherein the periodically polednon-linear optical crystal is one of lithium niobate, magnesium-oxidedoped lithium niobate, stoichiometric lithium tantalate, magnesium-oxidedoped stoichiometric lithium tantalate, and potassium titanyl phosphate(KTP).
 4. The laser of claim 1, wherein the down-converted signal has asignal wavelength of approximately 1380 nm to 1612 nm.
 5. The laser ofclaim 4, wherein the down-converted signal has a signal wavelength ofapproximately 1416 nm.
 6. The laser of claim 1, wherein the laser is oneof a continuous-wave laser, a Q-switched laser, a mode-locked laser, ora quasi-continuous-wave laser.
 7. The laser of claim 1, wherein theoptical parametric generator includes an optical parametric amplifier oran optical parametric oscillator.
 8. The laser of claim 1, wherein thefundamental laser includes a laser diode or a fiber laser.
 9. The laserof claim 1, wherein the fundamental laser includes a cesium lithiumborate (CLBO) crystal.
 10. The laser of claim 1, wherein the frequencymixing module includes a cesium lithium borate (CLBO) crystal, a betabarium borate (BBO) crystal, or a lithium triborate (LBO) crystal. 11.The laser of claim 1, wherein the frequency mixing module comprises ahydrogen-annealed non-linear crystal.
 12. An inspection systemcomprising: a laser for generating an output wavelength of approximately193.4 nm, the laser comprising: a fundamental laser; an opticalparametric generator, coupled to the fundamental laser, for generating adown-converted signal; a fourth harmonic generator, coupled to theoptical parametric generator or to the fundamental laser, for generatinga fourth harmonic; and a frequency mixing module, coupled to the opticalparametric generator and the fourth harmonic generator, for generating alaser output at a frequency equal to a sum of the fourth harmonic andtwice a frequency of the down-converted signal, wherein the frequencymixing module comprises two non-linear crystals configured so that onenon-linear crystal performs type-I frequency summation and anothernon-linear crystal performs type-II frequency summation, and wherein theinspection system further comprises a detector and optics configured tosimultaneously collect reflection and transmission images using thedetector.
 13. The inspection system of claim 12, wherein the inspectionsystem is a dark-field inspection system.
 14. The inspection system ofclaim 12, further comprising at least one acousto-optic modulator orelectro-optic modulator to reduce coherence of illumination.
 15. Theinspection system of claim 12, further comprising a pulse ratemultiplier to increase a pulse repetition rate.
 16. The inspectionsystem of claim 12, further comprising components for simultaneouslycollecting reflection and transmission images using same detectors. 17.The inspection system of claim 12, further comprising components forforming an illuminated line on a target being inspected.
 18. Theinspection system of claim 12, further comprising components for formingmultiple, simultaneously-illuminated spots on a target.
 19. Theinspection system of claim 12, wherein the fundamental laser generates afundamental frequency of approximately 1064.3 nm, approximately 1053 nm,approximately 1047 nm, or approximately 1030 nm.
 20. A laser forgenerating an output wavelength of approximately 193.4 nm, the lasercomprising: a fundamental laser; a frequency doubling module, coupled tothe fundamental laser, for generating a second harmonic; a frequencycombiner, coupled to the frequency doubling module, for generating athird harmonic; an optical parametric generator, coupled to one of thefrequency doubling module and the frequency combiner, for generating adown-converted signal; and a frequency mixing module, coupled to theoptical parametric generator and the frequency combiner, for generatinga laser output of a frequency equal to a sum of the third harmonic andtwice a frequency of the down-converted signal, wherein the fundamentallaser generates a fundamental frequency of approximately 1064.3 nm,approximately 1053 nm, approximately 1047 nm, or approximately 1030 nm.